Piracy of Android apps is common in 3rd party app stores. Pirates hack legitimate apps and redistribute them as their own. Four men in Florida and Oregon have been charged with such piracy and appear to be cooperating with the authorities, according to the WSJ story.

A recent study by security software firm Bitdefender indicates that even within the official Google Play store, "more than 1 percent of some 420,646 apps are stolen from other developers and re-engineered for illicit gains." The study's standard was that more than 90 percent of the app code, other than library code, had to be identical to that of a legitimate app.

The usual procedure in such cases is for copyright holders to send cease-and-desist letters and then to file a civil suits. In this case, the Justice Department says that "These crimes involve the large-scale violation of intellectual property rights in a relatively new and rapidly growing market."

Also being prosecuted are operators of app stores which hosted pirated apps. Two examples in the story are Aaron Buckley, 19, from Mississippi and Kody Peterson, a 22-year-old from Florida.

[This story was updated to add the reference to the Bitdefender study.]